The present invention relates generally to propulsion systems and, more particularly, to pulsed detonation engines used, for example, in aircraft and missiles.
In a conventional pulsed detonation engine (xe2x80x9cPDExe2x80x9d), a detonation chamber is filled with a fuel-air mixture capable of sustaining a detonation. A detonation wave is initiated in a small initiator tube connected to the detonation chamber and is introduced into the chamber. The detonation wave processes all of the fuel-air mixture within the detonation chamber as it propagates down the longitudinal axis of the chamber at supersonic speeds, resulting in high temperature, high pressure combustion gas that provides energy to drive the wave. The detonation wave eventually exhausts through an open end of the chamber at high velocity. As the detonation wave propagates through the chamber, a high pressure gas develops behind the wave that pushes against the rear surface of the detonation chamber and produces thrust. The detonation chamber is then refreshed with new air and fuel, pushing out the remaining exhaust products, and is prepared for another cycle.
Conventional PDEs heretofore known are generally incapable of generating sufficient thrust to sustain flight throughout the entire envelope of modem aircraft and missiles. The thrust performance for a fixed volume PDE may generally be increased in two waysxe2x80x94by increasing the repetition rate (i.e., the frequency of the pulses or engine cycles) or by increasing the impulse (thrust) per pulse. The pulse frequency is typically limited by the time required to refresh the detonation chamber with oxidizer (air) and fuel between each cycle, and is generally limited by the available cross-sectional area of the fuel and air inflow ports and the air and fuel flow rates. The maximum refresh flow rate may further be limited by choking at the minimum inflow area, i.e., limited by the physical dimension of the ports or tubing used in the refresh path. By developing a method to increase the minimum inflow area, a decrease in the amount of time required between cycles of a PDE can be achieved, thus increasing the thrust performance of the PDE.
In conventional PDEs, the cycle efficiency or impulse per pulse (and therefore, fuel efficiency) reflects the extent to which the inflow total pressure is converted to increase the pre-detonation pressure within the chamber in addition to reflecting the efficiency with which the thermodynamic energy of the high pressure detonated gas is converted into flow kinetic energy as it exits the engine. The greater the temperature of the combustion gases in the chamber, the higher the temperature of the exhaust gases as they exit the engine. This latent heat represents a loss in thermodynamic efficiency of the engine. Reduction of the temperature of the combustion gases while maintaining total cycle energy would improve the engine thermodynamic efficiency.
Another difficulty encountered with conventional PDEs is in the ability to efficiently and effectively transition the detonation wave from the initiator tube into the detonation chamber. The typical initiator tube used in a pulsed detonation engine is an elongated tube having a small diameter (e.g., one inch,) circular cross-section filled with a fuel-oxidizer mixture that is enhanced (e.g., by the use or addition of oxygen) to have a small critical diameter. The critical diameter of a detonable mixture is defined as the minimum diameter of an initiator tube required for the exiting detonation to directly transition into a spherical detonation wave in an open space filled with the same mixture. In a conventional PDE, the initiator tube is connected on one end to the interior of the detonation chamber. The downstream detonation chamber has a larger cross-sectional area and volume as compared to the initiator tube and conventionally contains fuel and air to power the detonation wave as it propagates through the chamber. The ability of the detonation wave to successfully transition from the initiator tube into the detonation chamber is dependent upon a number of factors, including the critical diameter of the enhanced fuel and oxidizer mixture contained in the initiator tube, the internal diameter of initiator tube, and the cross-sectional area of the detonation chamber. Generally, the larger the diameter of the initiator tube, the easier it will be for the detonation wave to transition from the tube into the detonation chamber having a larger cross-sectional diameter. However, as the diameter of the initiator tube is increased, the length of the tube must be proportionately increased to permit the spark-initiated flame in the tube to transition from deflagration (i.e., subsonic combustion) to detonation, resulting in an overall increase in the size and weight of the PDE. Therefore, it is preferable to maintain the diameter of the initiator tube as small as possible. However, it becomes increasingly difficult to transition a detonation wave into a downstream detonation chamber from a small initiator tube. One proposed solution for transitioning the detonation wave from an initiator tube into a larger detonation chamber is to use a divergent section that connects the initiator tube to the larger, constant cross-sectional area detonation chamber. However, conventional transition sections require large amounts of oxidizer to maintain the detonation wave until it finally reaches the full detonation chamber. This additional oxidizer flow equates to a lowering of the cycle efficiency of the PDE.
Thus, there is a continuing need for an efficient and effective pulsed detonation engine having increased efficiency over prior known PDEs. Preferably, the PDE would be configured to properly transition the wave (i.e., maintain the detonation wave from the initiator tube into the detonation chamber) with a low amount of enhanced fuel/air mixture. The preferred PDE would also have features designed to increase the thrust performance by decreasing the amount of time required to refresh the engine and/or increasing the amount of incremental thrust delivered each cycle.
A preferred pulsed detonation engine having these features and satisfying these needs has now been developed. In accordance with the presently preferred invention, an initiator tube is fueled with an enhanced fuel and oxidizer mixture having an associated small critical diameter and is in fluid communication with a detonation chamber via a divergent inflow transition section. The divergent inflow transition section has a diverging contoured shape having a rate of divergence continuously dependent upon the diameter of the tube, the critical diameter of the enhanced fuel mixture within the tube and the cross-sectional area of the detonation chamber. The preferred PDE also includes means for admitting a detonable fuel-air mixture into the transition section and the detonation chamber through the divergent inflow transition section. The preferred PDE also has means for igniting the enhanced fuel mixture contained within the initiator tube to create a detonation wave propagating through the initiator tube and transitioning over the divergent inflow section into the detonation chamber. The detonation wave generates a high pressure gas behind the wave that expands and produces thrust in a direction opposite to the exhaust.
In a preferred embodiment of the presently preferred PDE, the inflow transition section includes a plurality of openings to permit the fuel and air to enter into the detonation chamber. These openings may have a stair-step configuration or are configured as a continuous, porous surface. The openings are distributed along the inflow transition section so as to both admit flow during the refresh phase, and to enable successful transition during the detonation phase. An air induction valve is located upstream of the inflow transition section. This valve is opened during the refresh portion of each cycle, admitting the fresh charge of fuel and air. The valve closes prior to the passage of the detonation wave, holding in the detonation pressure and sustaining the thrust of the engine.
Alternatively, the air induction valve may be an integral part of the inflow transition section. In this configuration, the internal contour of the air induction valve, when closed, enables successful transition during the detonation phase. During the refresh phase, the valve opens to admit the fresh fuel-air mixture into the transition section and chamber.
In another embodiment, the pulsed detonation engine further includes an ejector/bypass flow volume. This mass of unfueled ejector/bypass air is contiguous to and in fluid communication with the fueled detonation mixture. The detonated fuel-air mixture expands, transferring energy from the high pressure, high temperature fueled flow to the unfueled ejector/bypass air, thus increasing the amount of energy from the detonation wave that is converted into thrust. The volume of the ejector/bypass air may be arranged in series with the fueled air (i.e., in front of or behind the fueled volume along the axis of travel of the detonation wave) or the bypass air may lie parallel to (i.e., to the side of or surrounding) the fueled volume. The volume of ejector/bypass air is produced and controlled either by the use of additional air induction valves, or by controlling the location and timing of fuel injection.
In another embodiment, the pulsed detonation engine includes a back pressure device which feeds inlet plenum air into the detonation chamber near its outlet end. Aerodynamic interactions with the back pressure flow slow the chamber refresh flow rate and thus back pressure the chamber. This improves the conversion of chamber inflow total pressure into pre-detonation chamber pressure, increasing impulse per pulse, and thus improving engine efficiency.